1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, which applies high voltage to a spark plug in the internal combustion engine.
2. Description of Related Art
Conventionally an ignition coil, in which high voltage is generated in a secondary coil by mutual induction effect, is known. An igniter interrupts electric power distribution to a primary coil, which constitutes a part of the ignition coil, to generate the high voltage. Then, the high voltage is applied to a spark plug that is connected to one end of the ignition coil.
The ignition coils are roughly classified into stick coils and rectangular coils. The stick coil has a stick-like shape, and its constituents such as a primary coil, a secondary coil, a central core and the peripheral core are arranged in a cylindrical case that is placed in a plug hole. The rectangular coil has a rectangular shape, and its constituents are arranged in a rectangular box-like housing that is placed above a top opening of the plug hole.
Recently, it is demanded to raise output of ignition coils. In a stick coil, the number of turns (outer diameter) of the primary and secondary coils and an outer diameter of the central core are limited by a bore diameter of the plug hole. In contrast, in a rectangular coil, the number of turns (outer diameter) of the primary and secondary coils and the outer diameter of the central core are not limited by the bore diameter of the plug hole and can be arbitrary determined. Accordingly, the need for rectangular coils has been growing instead of stick coils.
As an implementation of the rectangular coil, JP6-084664A discloses an ignition coil in which a columnar cylindrical central core, a primary coil, a secondary coil and a hollowed rectangular peripheral core are arranged in a housing. The hollowed rectangular peripheral core is opposed to both longitudinal end surfaces of the central core and is also opposed to both radial sides of the central core.
However, compared with a stick coil, in which constituents such as a central core is arranged in a cylindrical member, a rectangular coil, in which constituents such as a central core is arranged in a housing, has a relatively large height of the housing that is placed above an opening of a plug hole. However, it is demanded to reduce the height of the housing of the rectangular coil, that is, to downsize the ignition coil, since there is a necessity to reserve a space behind a hood of a vehicle for the purposes of pedestrian protection etc. In this regard, it is difficult to downsize the ignition coil in basically maintaining the shapes of the constituents such as a central core and a peripheral core, which are necessary for securing the performance of the ignition coil.